Interest continues to grow in the application of Multiport Amplifiers (MPAs) to Ku and Ka-band satellite payloads—See for example, A Couchman, D. Jones, “Optimized Multiport Amplifiers for Wideband Multi-beam Satellites”, AIAA, 24th International Communications Satellite Systems Conference, San Diego, USA, June 2006; A. Anakabe, et al, “Ka-band Multi-port Amplifier Characteristics for Space Telecommunication Operation”, 6th International Vacuum Electronics Conference, Noordwijk, Netherlands, April, 2005.
MPAs are particularly applicable to Single Feed per Beam (SFB) architectures which generate a set of regular contiguous beams over a defined coverage region, using a system of typically 4 antenna reflectors with associated feedhorns. The beam set employs a high degree of frequency re-use, with each beam generated uniquely by a corresponding feed horn. Such architectures are comparatively highly efficient in terms of RF power generation. However, they are much restricted in the flexibility with which they can distribute this power over the coverage area. The application of MPAs, with each feedhorn driven by a respective MPA output, would greatly increase the flexibility of the SFB design, significantly enhancing the flexibility of this architecture by enabling capacity (transmitted power) to follow dynamically changes in traffic distribution over the coverage zone. MPAs could be used in wideband, selectable bandwidth transponders providing flexible allocation of power as well as bandwidth to all beams ensuring optimum link parameters in each case.
An MPA is a well-known power amplifier device used for satellite communications, which operates at the microwave frequency bands. An MPA includes a number N of similar amplifier units (TWT or solid state) in parallel, each having a power P, so that each input signal is amplified equally by each amplifier, to increase the power of each output signal by a factor N, to P×N. N input ports and N output ports are provided, so that an input signal on one input port is routed to the corresponding output port. The input ports are connected to the amplifier units by a low power input network (INET) that may be implemented in any convenient transmission line technology that is appropriate to the circumstances, e.g. microstrip, stripline, coaxial cable, or waveguide. The output ports are connected to the amplifier units by a high power output network (ONET) that is implemented typically using low loss transmission line technology. The ONET is mathematically a reciprocal of the INET, so that a signal presented to the nth input is directed to the nth output. Each network comprises an array of signal dividing waveguide devices. Butler matrices or networks comprising just hybrid devices are normally used for signal division, because they have convenient gain and phase shift properties. A hybrid is a four port signal dividing device comprising two inputs and two outputs, with selective 90° phase shifts; this phase difference may be exploited to improve the isolation characteristics of the networks. However other hybrids and other signal splitting devices may be used which may have 180° phase difference.
The great advantage of an MPA is that in providing access for each input port equally to each amplifier, the accessible power available to each port is N×P, where P is the power of each individual amplifier. Thus the MPA embodies a high degree of flexibility, providing a wide range of output power which can be shared dynamically and in a highly flexible manner between the N inputs (or downlink beams). However a concomitant problem with an MPA is that of cross-talk between MPA output ports, and in general a lack of isolation between signals routed through the MPA.
MPAs, which have been considered for use in multibeam satellites for some time, have been successfully used at L-band and S-band (1.5-2.6 GHz): see S. Egami, M. Kawai, “An Adaptive Multiple Beam System Concept” IEEE Journal on Selected Areas in Communications, Vol. SAC5, No. 4, May 1987. M. Mallison, et al, “Advanced Payload for Multibeam Satellites that Support High Data Rate Broadband Global Area Network”, AIAA, 23rd International Communications Satellite Systems Conference, Rome, September 2005. M. Tanaka, et al, “S-band Multibeam Transmitter for N-STAR”, AIAA, 16th International Communications Satellite Systems Conference, Washington, USA, February 1996.
However, these operate at wavelengths which are around a factor of ten longer than those at Ku and Ka-bands (12-20 GHz). The problems of phase and amplitude misalignment of individual amplifiers of an MPA at the Ku/Ka-bands, and hence that of isolation and signal combining performance, become considerably greater and may bring the feasibility of operating MPAs at these frequencies onboard a satellite and over the required service life into question.
U.S. Pat. No. 7,088,173 discloses a method for tuning phase relationships for an MPA, including selecting one of a plurality of test patterns which detects phase information of an amplifier unit of the MPA, detecting an output signal of the test pattern at a designated MPA output, and adjusting a phase relation of the amplifier unit based upon the output signal.